Preserve order of hashes or sort them on write in requirements file

[matejsp]

As per reported issue in pipenv pypa/pipenv#5552 there is issue that requirementslib stores hashes in non deterministic order. They should be stored in alphabetic order or in preserve order based on insertion.

Method add_hashes produces a set with no order guarantees.
https://github.com/sarugaku/requirementslib/blob/main/src/requirementslib/models/requirements.py#L2427

@matteius already prepared a pull request that sorts hashes on write.
#357